The present invention, in some embodiments thereof, relates to hydrogen production and, more particularly, but not exclusively, to novel systems and methods for performing hydrogen production by water electrolysis.
The issue of renewable energy has become the focus of many researches over the past few decades. Energy demand is increasing, and is projected to increase even faster, driven by strong economic growth and expanding populations. This energy is largely supplied by fossil fuels (82% according to the EIA data). This presents several challenges, mainly the resulting pollution and depletion of natural resources.
These challenges bring about the need for energy systems which are based on renewable resources. Specifically, many efforts have been focused on solutions for electricity generation and incorporation of such systems into the electrical grid. A prominent example of such a system is photovoltaic electricity production.
However, an electric system based on renewable energy faces the issue of intermittency, with the main challenge being the mismatch between the time periods of energy production and energy demand. This mismatch poses a threat to grid stability and creates a barrier for the incorporation of renewable energy into the grid. In the case of photovoltaic electricity, solar energy is only provided during daytime and may further be disturbed by undesirable weather conditions.
A promising pathway for overcoming this obstacle is the conversion of renewable energy into synthetic fuel. In this respect, one of the main candidates being investigated as a renewable fuel is molecular hydrogen (H2). While molecular hydrogen is scarcely found in nature, it is stored in vast amounts in water molecules, and it can be released from water by electrolysis [1] [2] [6] [7]. See, for example, Krol, R. and Gräzel, M. (2012). Photoelectrochemical hydrogen production. 1st ed. New York: Springer.; Bak et al. (2002) International journal of hydrogen energy, 27(10), pp. 991-1022; Pinaud et al., (2013) Energy & Environmental Science, 6(7), pp. 1983-2002; Ursua et al. (2012) Proceedings of the IEEE, 100(2), pp. 410-426.
Electrolysis is the process wherein electric current passes through an electrolyte resulting in chemical reactions that decompose the electrolyte. In water electrolysis, an external power source is connected to two electrodes which are immersed in an aqueous electrolyte. The electrodes are typically made from an inert metal and the reactions taking place are oxidation and reduction of aqueous species within the electrolyte. Electric current causes ions to migrate to the oppositely charged electrode where either a reduction or oxidation reaction takes place. In widely used alkaline water electrolysis, the electrolyte is an alkaline solution, usually a concentrated solution of KOH or NaOH. The hydroxide ions (OH−) migrate to the anode where the oxidation evolution reaction (OER) takes place and oxygen gas (O2) is evolved. Hydrogen gas (H2) is evolved at the cathode where water reduction, or hydrogen evolution reaction (HER) takes place. The overall reaction is:H2O→H2(g)+½O2(g).
RuO2 and IrO2 are currently considered optimal materials for OER because they exhibit the lowest overpotentials. However, because the RuO2 and IrO2 are expensive and have poor long-term stability in alkaline solution, oxy-hydroxide films of nickel and its alloys are more frequently used for OER anodes [Lyons et al., Int J Electrochem Sci 2012, 7:2710-1763].
Another component of most water electrolysis systems is the membrane which is necessary to prevent the mixing of product gases with one another for the sake of efficiency and safety. While the membrane separates the reaction products, oxygen and hydrogen, it allows the transfer of ions, thereby facilitating ionic current between the anode and cathode.
Currently practiced technologies which employ the use of a membrane for alkaline water electrolysis are typically based on the concept of a single cell within which both reactions take place, wherein the cathode and anode compartments are separated by a membrane. This configuration, however, is incompatible when, for example, substances such as photo-electrodes are introduced into the system.
Within the last four decades, photoelectrodes have been studied extensively and they are considered promising for sustainable hydrogen production systems, in technologies for water electrolysis that employ photoelectrochemical (PEC) cells.
A PEC cell consists of a semiconductor photo-anode and metal cathode and/or a semiconductor photo-cathode and metal anode. When the semiconductor photo-electrode is illuminated with light having energy greater than its band-gap, an electron is excited from the valence band to the conduction band, creating an electron-hole pair.
In the case of alkaline water electrolysis using a metal cathode and a semiconductor photo-anode, holes accumulate at the photo-anode/electrolyte interface where the oxygen formation takes place. The electrons are transported via the connecting wire to the metal cathode where hydrogen formation takes place. Hydroxide ions migrate through the electrolyte in the opposite direction, namely, from the cathode to the anode, thereby closing the current loop wherein electrons travel in the external circuit (through an electric wire that connects the anode to the cathode) and ions travel through the electrolyte.
The PEC water splitting into H2 and O2 entails the development and exploration of semiconducting materials which are chemically stable and have significant optical absorption cross section. Since the discovery of water photolysis on TiO2 electrodes by Fujishima and Honda, semiconducting metal oxides have remained under focus [see, for example, Sivula et al. (2011). ChemSusChem, 4(4), pp. 432-449]. Among the various oxides, hematite (α-Fe2O3), a stable, non-toxic, and abundant material which is photoactive under visible light, has been the subject of much interest [see, for example, U.S. Pat. No. 6,228,535].
One issue which arises from the use of a photo-electrode is the large area needed in order to collect sunlight since the photocurrent density at the photo-electrode is much smaller than the current density at the counter metal electrode.
Therefore, the size of the photo-electrode compartment must be much larger than that of the counter metal electrode. This leads to a major difficulty in the aforementioned single-cell configuration because of the need to seal the entire assembly. A solar PEC hydrogen production plant requires the entire array to be totally sealed in order to collect the hydrogen gas. This is a very difficult technical challenge given the huge area of the PEC solar collectors, and moreover the nature of the produced H2 which is unsafe, difficult to encase, and requires special materials to contain it.
The production, storage and transportation of hydrogen gas pose additional obstacles. Hydrogen gas is highly diffusive; it has an extremely low density and a broad flammability range. Hydrogen production in the same cell where oxygen is produced presents a safety problem in addition to the possible decrease in system efficiency. These characteristics of hydrogen gas lead to essential difficulties at every step from distribution through storage to end-usage. As a partial solution, hydrogen can be stored and transported using carbon or nitrogen carriers which are safer and easier to handle than hydrogen. In any event, in a configuration where hydrogen and oxygen are produced in a single cell on the solar array site, hydrogen needs to be transported from this site to a distribution network or to a fuel production site.
Additional background art includes International Patent Application PCT/US2006/014122 (published as WO 2006/113463) and U.S. Patent Application Publication No. 2012/0121998.